CHARACTERISTICS OF CENTRIFUGAL PUMPS
Flow Head or Q-H curve characteristics. the shape of the Q-H curve
Allowable operating range.
Power curve (BHP, P2) the shape of the Power curve. The shape of the power curve.
Efficiency characteristics. Lines of the equal efficiency.
Pump characteristics in the catalogs. Special points worth taking account.
Flow Head (Q-H) characteristics
The Q-H characteristic shows how it depends pump Head from the pump Flow or how the Head will change when changing pump Flow.
It often comes as surprise that pump flow and pump head can be related. However, each pump has its own characteristic and the relation is different for different pumps.
Lets look at a simple way of describing this: If the hose is lifted high above the pump [position HI] then very little flow [Q1 ] will drip out of its end. Position h0 has a special term: it is called the 'shutoff head' or 'closed valve head'. If the hose is now lowered a little to position H I , more flow comes out of the pipe [Q2]. This process can then be repeated at several heads [Hx] and associated flows [Qx] measured. [...]
The characteristic obtained in this way is called flow head characteristic of a centrifugal pump.
The pump can operate at any points lying on it.
Different head values correspond to different flows. As a rule, with increasing head decreases flow and vice versa, as the flow increases, the pressure decreases.
Pump can operate at any point of it's Q-H characteristic.
But Some pump users think that pump can provide parameters which were printed on the nameplate not more and not less.
The data on the pump nameplate refer to one duty point only.
What determines the head of the pump?
The larger the impeller diameter, the higher the fluid velocity, the higher the pump Head.
What determines the flow rate of the pump?
The wider the impeller channel, the more liquid passes through the impeller more pump flow.
The pump can operate at any point of the curve from zero flow up to the maximum.
But pump manufacturers limit the minimum and maximum flow. Why?
Pump operating range
1 - allowable operating range of feed
2 - preferable operating range
3 - maximum vibration level for allowable operating range
4 - maximum vibration level in preferred operating range
5 - Q bep Flow corresponding to the point of maximum efficiency
6 - curve of dependence of the average vibration on flow, which shows the maximum allowable vibration
7 - Q-H characteristic of the pump
8 - point of maximum efficiency, Head and Flow
The allowable operating range of the pumps must be between 70% and 120% of the flow, corresponding to the maximum efficiency of the pump.
The preferred operating range is 80% to 110% of the flow corresponding to maximum pump efficiency. API 610 "Centrifugal pumps for oil, petrochemical and gas industry.
For pumps for common industry during normal operation, a good rule of thumb is that a centrifugal pump should always be operated between 50% and 120% of the BEP.
The reason for this range is based on the physics of the design. Operation in this zone ensures the highest efficiencies and the most reliable operation, with the lowest bearing loads, turbulence and
Why vibration level rises when the operating point moves beyond the allowable operating range?
Vibration in the pump is caused by the fact that the flow patter unstable, vortices and pressure distribution are formed. At the point of maximum efficiency, the liquid in the pump flows without turbulence and pressure pulsations. When going outside the operating range, the unevenness of the fluid flow is violated all more it leads to a drop in efficiency and vibration.
In the pump case the impellers of different diameters can be installed.
Depending on the diameter the pump head also changes and we receive different Q-H characteristics.
In the Manufacturer's Catalog at the beginning you can see the field of characteristics of the entire standard hydraulic range or family range, which consists of the Hydraulic ranges or of pump models.
Each field is limited by the left and right borders of the allowable operating range and by the maximum and minimum impeller diameters.
Shaft Power characteristic
The power characteristic (Q-Power) shows how the power absorbed by the pump changes depending on the flow. As in the case of the head characteristic, the power characteristic is built for a fixed speed.
The shaft power is, just as the name suggests, the power that is transmitted from the motor through the coupling, to the pump shaft. Like the head curve, this curve is generally plotted for a fixed operating speed.
Note that the shaft power is not the motor input power, which can usually be measured in the field, but the motor output power, which generally cannot.
However, a good estimate of shaft power can be developed if the motor efficiency at the different load conditions is known.
What is necessary to remember:
- this is the power on the pump shaft not electric power.
- the electric power consumed by the electric motor depends on the pump power. The shaft power of the pump and electric motor are equal.
Some users think that electric motor consumes the nominal power printed on the nameplate.
In most cases, the pump power increases as the flow increases. More power is needed to pump more fluid. But this is not always fair. There are cases when the power characteristic may, upon reaching a certain value, with a further increase in feed, decrease.
Please note that the minimum pump power is at zero flow. Therefore, the centrifugal pump is started either on a fully closed or ajar gate valve.
The shape of the power characteristics can also have a gently dipping form. This shape of the characteristic is typical for pumps with a large flow and low head. Axial or semi axial pumps.
The shape of the power curve
Rising characteristic. In this case, selection of power of electric motor should be more careful..
There is a risk of the motor overloading.
BEP - Q=320 m3/h H=80 m n=1450 rpm.
There is no risk of motor overloading
BEP: Q=1250 m3/h H=25 m n=1450 rpm.
The steepness of the curves plays an important role in selection of the pumps methods of pumps control, influence on pump parameters after wearing. The steepness of the Q-H and Power characteristics are connected.
Look at the picture below.
1. Q-H has a less steepness (flat) Power has a rising shape.
2.Q-H has more steepness Power has not rising shape.
Pump efficiency characteristic
The efficiency of the pump, like any other mechanism is the ratio of useful power to consumed power.
Pump manufacturers, through performance testing, determine the efficiency for each model pump. This point is called the Best Efficiency Point (BEP). BEP should be stated on all centrifugal pump curves. At the BEP, the least amount of fluid is bypassed back to the low-pressure (or suction) side.
BEP (Best Efficiency Point. can be only one on the curve.
Pump efficiency characteristic shape
The shape of the efficiency curve can also vary significantly from pump to pump. As shown in the figure, it can have a pronounced peak at the point of maximum efficiency and then sharply decrease as the feed is increased or decreased. Conversely, the characteristic may have a stretched high efficiency region without a pronounced peak. Taking into account the fact that the pump almost never works at one point, it is very difficult to accurately select a pump for the requirements of the system. Then, from the point of view of operation, the option with a characteristic that has a wide range of high efficiency looks more preferable.
Pump efficiency in catalogs
In catalogs, efficiency characteristics can be presented either as a separate curve or as lines of equal efficiency plotted on the Q-H characteristic. There is only one point of maximum efficiency, so there is not much point concentrate on her. The pump almost never operates at this point.
This representation is convenient from the point of view of impact analysis impeller diameter. With a decrease in diameter the efficiency of the pump decreases slightly. This reduction is compensated to a greater extent by the fact that with a reduced impeller diameter the pump characteristics correspond to the requirements of the system and the pump operates in the mode that is closest to optimal.
Special points on the curve.
Let's go into more detail consider the characteristics that are given in the catalogs manufacturers. What to look for when choosing pumps. The figure shows a typical pump head characteristic from the catalog of one of the manufacturers.
Pump characteristics are given for a specific frequency rotation. The actual pump speed at the site will be
different from the one shown in the catalog. Therefore, when conducting an assessment on an object, it is necessary to bring the characteristics to the actual speed that was measured on the object or on a test bench.
For example, this characteristic is given at a frequency of 1450 rpm.
The characteristics in the catalogs are given on the water under normal conditions.
If the pump pumps liquids with other physical properties, viscosity, density, then it is necessary to adjust the characteristics. We will consider in more detail the effect of fluid properties on pump characteristics separately. For example, the high viscosity of the liquid increases the power consumption, which must be taken into account when choosing the power of the electric motor.
There are several points and areas on the characteristics be taken into account when selecting a pump.
1. Point of maximum efficiency. And the corresponding pump flow.
2. Right and left border of the working range. This area in which the pump is allowed to operate. Need to pay
attention to other pump characteristics at extreme points operating range. For example, what will be the power on the right limit, as a rule, this is the maximum power on the pump shaft, which is necessary to select the power of the electric motor.
Power at zero flow of the pump is required for selection electric motor.
3. Point of maximum head. Most often this is head on a closed valve at zero flow (shut off head). This value is necessary for to determine the maximum pressure in the hydraulic system and, accordingly, based on this pressure, pipelines are selected and fittings, but it must be remembered that when determining maximum pressure in the system, it is necessary to take into account the pressure at the pump inlet. The maximum pressure is defined as the sum pressure at the inlet to the pump and the pressure of the corresponding maximum pump head.
The example of pump characteristics presented in a catalog of pump manufacturers.
Pump pressure or pump head is the difference between pressure or head at the pump inlet and outlet.
Sometimes users forget about inlet pressure and consider only pressure in the outlet.
More detail information find on the page Cavitation
Q-NPSH characteristic shows how NPSH depends on the pump capacity.
NPSH (Net Positive Suction Head ) is the parameter which characterizes the suction capability of a pump.